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This invention relates to induction well logging, and more particularly to a new transmitter and receiver coil structure, and a new method for collecting and processing data from an induction tool.
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as xe2x80x9clogging,xe2x80x9d can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a petrophysicist regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or xe2x80x9csondexe2x80x9d is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed cylinder for housing the sensors, which hangs at the end of a long cable or xe2x80x9cwireline.xe2x80x9d The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth""s formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
The sensors used in a wireline sonde usually include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors.
If the formation properties are needed while drilling, sensors can also be deployed near the end of a drilling string. Measurements of formation properties can be measured and stored in memory for later retrieval and correlation with depth. Measurements can also be transmitted to the surface by pulses of mud pressure or other means. This process is referred to as xe2x80x9clogging while drillingxe2x80x9d (LWD).
For a formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well-known physical characteristics. Hydrocarbons are a poor conductor of electricity while most formation water conducts much better. If the porosity of an earth formation is known from other sensors, its electrical resistivity can assist the petrophysicist in determining the volume fraction of hydrocarbons in the formation. This electrical resistivity can be measured by two classes of sensorsxe2x80x94those that use electrodes to force current to flow through the formation and to measure potential differences, and those that use coils to induce currents to flow magnetically. The particular type with coils, called induction devices, determine electrical resistivity by inducing an alternating electromagnetic field into the formation with a transmitter coil arrangement. The electromagnetic field induces alternating electric (or eddy) currents in the formation in paths that are substantially coaxial with the transmitter. These currents in turn create a secondary electromagnetic field in the medium, inducing an alternating voltage at the receiver coil. If the current in the transmitter coil is kept constant, the eddy current intensity is proportional to the conductivity of the formation. Consequently, the conductivity of the formation determines the intensity of the secondary electromagnetic field, and thus, the amplitude of the voltage at the receiver coil. As will be apparent to one skilled in the art, the propagating electromagnetic wave suffers both attenuation and phase shift as it traverses the formation.
An exemplary induction tool is shown in the prior art drawing of FIG. 1, in which one or more transmitters (T) and a plurality of receivers (Ri) are shown in a logging sonde. Each transmitter or receiver is a set of coils, with modern array induction tools having several receivers of increasing transmitter-to-receiver spacing to measure progressively deeper into the formation.
In a conventional induction tool such as that shown in FIG. 1, the coils are wound coaxially around a cylindrical mandrel. Both transmitter coils and receiver coils are solenoidal, and are wound coaxial with the mandrel. Such coils would therefore be aligned with the principal axis of the logging tool, which is normally also the central axis of the borehole and is usually referred to as the z axis. That is, the magnetic moments of the coils are aligned with the axis of the mandrel on which they are wound. The number, position, and numbers of turns of the coils are arranged to null the signal in a vacuum due to the mutual inductance of transmitters and receivers.
During operation, an oscillator supplies alternating current to the transmitter coils, thereby inducing voltage in the receiver coils. The voltage induced in the receiver coils results from the sum of all eddy currents induced in the surrounding formations by all transmitters. Phase sensitive electronics measure the receiver voltage that is in-phase with the transmitter current divided by magnitude of the transmitter current. When normalized with the proper scale factor, this gives the apparent conductivity of the formation. The out-of-phase component can also be useful because of its sensitivity to skin effect although it is less stable and is adversely affected by contrasts in the magnetic permeability.
As noted, the induced eddy currents tend to flow in circular paths that are coaxial with the transmitter coil. As shown in FIG. 1, for a vertical borehole traversing horizontal formations, there is a general symmetry for the induced current around the logging tool. In this ideal situation, each line of current flow remains in the same formation along its entire flow path, and never crosses a bed boundary.
In many situations, as shown for example in FIG. 2, the wellbore is not vertical and/or the bed boundaries are not horizontal. The well bore in FIG. 2 is shown with an inclination angle xcex8 measured relative to true vertical. A bed boundary between formations is shown with a dip angle xcex1 relative to the axis of the borehole. The inclined wellbore strikes the dipping bed at an azimuth angle xcex2. As a result, the induced eddy currents flow through more than one medium, encountering formations with different resistive properties. The resulting logs tend to be relatively inaccurate, especially as the dip angle xcex1 of the bed boundaries become more severe. If the logging tool traverses a thin bed, the problem becomes even more exaggerated.
As shown in the graph of FIG. 3A, an induction sonde traversing a dipping bed produces a log with xe2x80x9chorns.xe2x80x9d The more severe the dip angle, the less accurate is the measurement. FIG. 3A represents a computer simulation of a log that would be generated during logging of a ten foot thick bed (in actual depth), with different plots for different dip angles. FIG. 3B shows a computer simulation of a log which would be generated if the thickness of the bed were true vertical depth, with different plots for different dip angles. As is evident from these simulated logs, as the dip angle increases, the accuracy and meaningfulness of the log decreases. In instances of high dip angles, the plots become virtually meaningless in the vicinity of the bed boundaries.
Formation anisotropy further complicates the interpretation of conventional induction tools. There are at least two major interpretation problems related to anisotropy. The first is in interpreting logs from a number of wells drilled from a common platform. Each well path intercepts the zone of interest at a different angle of relative dip. If the zone is anisotropic in resistivity, the zone will have a different measured resistivity that is a function of dip angle. This effect is present in thick beds. In thin beds, the problem is compounded with the polarization horns at boundaries and the change in spatial response with dip angle.
The second problem is the case of finely laminated sand/shale sequences. This is the so-called low resistivity pay problem. These zones can be productive if the thin, sand layers are saturated with oil. When water wet, the sands are electrically conductive and similar in conductivity to the shale, so the formation is reasonably isotropic. If the sands are saturated with oil, they act as insulators separating the conductive shale layers. Measured horizontally along the layers, the insulating oil layers are electrically in parallel with the conducting shale layers, and the shale conductivity dominates. The conductive shale layers xe2x80x9cshort outxe2x80x9d the resistive sand layers. Measured vertically across the stack of layers, the layers are electrically in series, and the high resistivity of the sand layers dominates. A conventional tool that measures only the horizontal resistivity will give a poor estimate of the oil saturation of the composite since it predominately sees the low resistivity of the shales. A tool that measures both components can better estimate the saturation. Therefore, it is desirable to measure formation anisotropy even in situations of low relative dip. Of course, relative dip further complicates this interpretation problem.
Many efforts have been made to develop induction well logging equipment and methods of operation to measure characteristics of materials surrounding well bores while avoiding these known problems, and various devices have been developed to measure the dip angle of bed boundaries to give more meaning to the logs. For example, it has been appreciated that the accuracy of induction logs could be improved if it were possible to keep the transmitter coil and receiver coils parallel to the bed boundaries (and also with each other). To accomplish this, it is known to provide separate transmitters and receivers, with each transmitter and receiver being comprised of an array of three separate coils. One of these three coils is aligned with the principal axis of the logging tool and therefore is aligned with the well. The other two are positioned perpendicular to the tool axis and to each other, such as is generally shown in FIG. 5. Due to the physical constraints of the space available in the logging tool, these additional coils cannot be of the same size and shape as the main coil wound on the mandrel. However, to facilitate measurements, the three coils are centered on the same point such as shown in FIG. 6. The vertical coil of FIG. 6 is simply wound around the mandrel as in the classic non-steerable tools. The horizontal coil is formed of a pair of saddle shaped coils placed on the surface of the mandrel. Another horizontal coil may be placed at right angle to the one illustrated to complete a triad.
The transmitter and receiver arrays may be selectively driven to create a virtual sonde, with the virtual coil arrays parallel to the bed boundaries to produce magnetic moments that are orthogonal to the bed boundaries. See generally U.S. Pat. No. 5,757,191 to Gianzero (incorporated herein by reference for all purposes). The coils are also steered to create a virtual transmitter and receiver perpendicular to the bedding planes to measure anisotropy. For example, FIG. 17 shows the effect of steering the array of coils to effect virtual transmitters and receivers that are parallel to the bedding planes to measure formation resistivity. Similarly, U.S. Pat. No. 4,360,777, issued to Segesman on Nov. 23, 1982, (incorporated herein by reference for all purposes) discloses an electronically steerable transmitter and receiver coil arrangements which allow the induced current loops to be aligned with the layered formations surrounding a well. Segesman adjusts the phase and amplitude of transmitter signals to the three transmitter coils to generate a composite transmitted signal which is aligned with the formations surrounding the well. See also Gianzero, S. and Su, S. M., xe2x80x9cThe Response of an Induction Dipmeter and Standard Induction Tool to Dipping Beds,xe2x80x9d Geophysics, Vol. 55, No. 9 (September 1990). Similarly, other devices have been developed to measure the strike angle between the wellbore and the bed boundary.
However, despite the usefulness of virtual sondes, problems still exist with these arrangements. For example, the non-planar windings of the saddle shaped coils of FIG. 5 cause field assymetries which could complicate the beam steering methods taught by Segesman and Gianzero. Also, the mechanical arrangement of the triads of coils makes them tedious to manufacture and install and can substantially weaken the device. Further, to create a virtual sonde that includes coils parallel and perpendicular to the bed boundaries, it is necessary to determine the angle of any bed boundary prior to driving the transmitter and receiver coils. The logging process requires repeated measurements of dip and adjustment of the virtual sonde angle, thus slowing the process. This also presents difficulties if the dip angle of the bed boundary is measured inaccurately.
The invention includes a new coil arrangement and method for performing induction logging of wells. Each coil arrangement comprises three separate coils each having one or more conductive loops, with each loop lying substantially in a plane tilted substantially from the principal axis of a logging tool. The three coils are positioned symmetrically about the axis of the logging tool and centered on about the same point. The arrangement places the axes of each of the three coils in orthogonal relationship to the other two.
The invention also includes an improved logging method. A set of coils is used to transmit a set of induction logging signals. At each desired receiver location, one or more coil arrangements are used to receive and record each of the three transmitted signals, for a typical total of up to nine signals. The recorded data may then be mathematically operated on to xe2x80x9csteerxe2x80x9d a virtual induction sonde after the data has been taken. This allows measurement of any of the properties of materials surrounding the borehole which have been measured by prior known induction logging tools.